1. Field of the Invention
The present invention relates to a spot size converter used as an optical link between a silicon-wire waveguide and an external optical device such as an optical fiber or laser diode.
2. Description of the Related Art
Silicon-based waveguide technology has been attracting attention recently. A silicon-wire waveguide, in which a silicon (Si) core is surrounded by a silicon dioxide (SiO2) clad having a much smaller refractive index, is one type of silicon-based waveguide. Because of the large difference between the refractive indexes of the core and clad, light is strongly confined in the core. This tight confinement enables a silicon-wire waveguide to turn corners with a radius of curvature as small as about one micrometer (1 μm). Silicon-wire waveguides can accordingly be used to create optical devices with submicron-order dimensions, which can be integrated with electronic devices on the same silicon chip.
Because the core diameter of a silicon-wire waveguide is much smaller than the core diameter of most optical fibers, optically coupling a silicon-wire waveguide to an optical fiber generally requires a spot size converter. The same is true of optical couplings between silicon-wire waveguides and other external optical devices such as laser diodes.
Many types of spot size converters have been proposed. Japanese Patent Application Publications (JP) H07-063935, 2000-235128, 2002-162528 (U.S. Pat. No. 6,937,797 to Mizuno et al.), and 2003-207684 (U.S. Pat. No. 7,099,540 to Shimoda) disclose optical waveguides with tapered widths, while JP 09-015435 and JP 2005-326876 disclose optical waveguides with tapered thicknesses. U.S. Pat. No. 6,684,011 to Jeong et al. discloses a spot size converter having a vertical double-layer structure in which the upper core layer is tapered in the width direction.
One problem with these known spot size converters is their large polarization dependency. Another problem is that many of them are extremely difficult to manufacture.